Sound producing device



July 1, 1958 E. cs. THURSTON Filed Aug. 8, 1955 5 Sheets-Sheet l fmconme g SIGNAL HIGH Low FRE E PRE-AMPLIFIEIZ SEPARATION CIRCWT f .20H\6H FREQUENCY AMPLIFIER LOW FREQUENCY 7*46 AMPLIFIER LOW PASS HLTER@zaewzfi" HwaW-d19.q%m/w77z July 1, 1958 E. 5. THURSTON 2,841,643

SOUND PRODUCING DEVICE 4 Filed Aug. 8, 1955 3 Sheets-Sheet 2 4 w p @wwww (Q Qwh y 1958 'E. G. THURSTON 2,841,648

SOUND PRODUCING DEVICE Filed Aug. 8, 1955 5 Sheets-Sheet 3 United StatesPatent SOUND PRODUCING DEVICE Edward G. Thurston, Chicago, 111.,assign-or to Electro- Voice, Incorporated, Buchanan, Mich.

Application August 8, 1955, Serial No. 526,864

Claims. (Cl. 1791) The present invention relates generally to soundreproducing devices, and more particularly to devices for reproducingsound from electrical signals with high fidelity.

The fidelity of sound reproduction from electrical signals has greatlyimproved in recent years, partly because of improved loud speakers andpartly because of improved devices for impressing the electrical signalsupon loud speakers. Most of the presently used systems of reproducingsound utilize an amplifier provided with an output transformer, and aloud speaker connected to the output transformer. It is well-known thatthe frequency response of the amplifier and the distortion from theamplifier may be improved by providing a degenerative feedback loop fromthe output of the transformer to the input of the amplifier.

The loud speaker is also coupled to the air in the room in which it isdisposed, and the frequency response of the speaker and the distortionby the speaker vary due to mechanical resonances, changes in thecoupling between the speaker and the air, etc. No known system offeedback compensates for these effects, and hence only the design of thespeaker can at present be employed to maxi mize the range of soundresponse and minimize the sound distortion from a given electricalsignal.

One of the objects of the present invention is to improve the frequencyresponse and distortion characteristics of loud speakers by providing anegative feedback loop from the output of the loud speaker to the inputof the amplifier of the sound reproducing device.

One of the criteria of negative feedback circuits is that the feedbackvoltage must always be out of phase with the signal voltage for properoperation. If the feedback voltage becomes in phase with the signalvoltage, an undesirable condition of sustained oscillation occurs. Inorder to provide a negative feedback circuit which is responsive to theacoustical output at the speaker, an electro-mechanical transducer mustbe disposed in the air adjacent to the speaker to generate thedegenerative feedback voltage. Since with any practical mountingarrangement the distance from the speaker to the electro-mechanicaltransducer must be fixed, the phase of the feedback voltage will dependupon the frequency of the incident sound wave upon theelectro-mechanical transducer. Thus as the frequency increases, thewavelength of the sound decreases and will approach in magnitude thefixed distance from speaker to electro-mechanical transducer. This willcause a progressive phase shift in the feedback voltage of thetransducer relative to the input signal voltage of the amplifier so thateven though the feedback and signal voltages were properly out of phaseat low frequencies, they may become in phase at higher frequencies andoscillation will occur. Thus, a negative feedback loop, which merelydisposes a microphone adjacent to the loud speaker and indiscriminatelyfeeds the output voltage of the microphone to the input of theamplifier, will produce oscillations. Furthermore, such oscillations canoccur at a frequency above the normal operating range of the speakerunless proper precautions are taken to avoid them, and such anoscillatory condition may not only cause an unpleasant and undesirablesound but will also render the en tire system incapable of high fidelityreproduction at any frequency.

It is a further object of the present invention to provide a soundreproducing device which employs an electromechanical transducerpositioned adjacent to a loud speaker and means to feed back theresponse of the electromechanical transducer to the input of the speakeramplifier which prevents the feedback from becoming in phase with thesignal to the input of the amplifier.

The inventor has found that it is possible to feedback a signalgenerated by an electromechanical transducer positioned adjacent to theloud speaker of a sound reproducing system if the frequency bandwidth ofthe feedback is limited. It is known that in the high fidelityreproduction of sound, it is usually the low frequency signals which arethe most ditficult to reproduce with a smooth response and lowdistortion. In the operation of low frequency, comer-loaded, horn type,loud speakers, such as that disclosed in the Patent No. 2,373,692 issuedto Klipsch, the frequency response is approximately from 30 cycles persecond to an upper limit of from 200 to 500 cycles per second and inthis frequency range, the shortest wave length to be encountered, as faras substantial radiation is concerned, is between 2 /2 and 5 /2 feet.The upper limit of this frequency response is not only limited by theability of the loud speaker and enclosure to substantially radiate soundat higher frequencies, but also by the fact that for high fidelityreproduction means are usually provided to purposely transfer electricalexcitation from the low frequency speaker to another speaker or otherspeakers whose design permits better reproduction of higher frequencysounds. In general, it is desirable to cause the frequency of transferof electrical excitation, or so called crossover-frequency, to besubstantially below the frequency at which the effective radiation ofthe low frequency speaker starts to decrease due to its own performancelimitations at high frequencies and likewise substantially above thatfrequency at which the response of the high frequency speaker starts todecrease due to its inherent limitations at low frequencies. Inpractice, the low frequency speaker is usually capable of substantialradiation for at least one octave above the crossover frequency, and thehigh frequency speaker capable of substantial radiation for at least oneoctave below the crossover frequency.

The inventior has found that an electro-mechanical transducer,positioned from the vertex of the low frequency loud speaker by adistance not to exceed and preferably less than A; of a wavelength ofsound in air at the crossover frequency of the low frequency speaker,may be used to generate a negative feedback signal which is fed into theinput of the amplifier driving the speaker Without introducingsufiicient phase shift at any frequency to cause oscillations todevelop, provided the response of the feedback circuit is properlyattenuated above a frequency equal to three to four times the crossoverfrequency of the low frequency speaker.

The invention will be more fully understood, together with additionalobjects and advantages, from a further reading of this specification,particularly when viewed in the light of the drawings, in which:

Figure 1 is a schematic diagram of a device for reproducing sound froman electrical signal;

Figure 2 is a schematic electrical circuit diagram of the high and lowfrequency separation circuit and high frequency amplifier shown inFigure l; and

Figure 3 is a schematic electrical circuit diagram illustrating the lowfrequency amplifier and feedback circuit shown in Figure 1.

The construction of the invention illustrated in the figures employs aplurality of speakers, such as a high frequency speaker and a lowfrequency speaker 12, the high frequency speaker 10 being connected tothe output of a high frequency amplifier 14, and the low frequencyspeaker 12 being connected to the output of a low frequency amplifier16. A high and low frequency separation or crossover circuit 18 hasseparate outputs connected to the high frequency amplifier 14 and thelow frequency amplifier 16, and a pre-amplifier 20 is connected to theinput of the high and low frequency separation circuit 18. The incomingelectrical signal is impressed upon the input of the pre-amplifier 20,this signal being supplied by a high fidelity signal source, such as atape recorder, high frequency tuner, or the like.

The low frequency speaker 12 is disposed within a speaker enclosure 22of the type described in the above referred to patent. The enclosure 22has a solid front panel which is adapted to be disposed diagonallyacross a corner of a room. A pair of side panels 26 are attached to theedges of the front panel 24, a space being provided between the sidepanels 26 and walls of the room. The side panels 26 are secured to arear support strip 28 which is adapted to be disposed in contact withthe walls of the room at the corner thereof, and a cover 30 and base 32extend from the rear support 28 to the front panel 24. A speaker housing34 with an inner panel 36 confronting and parallel to the front panel 24is secured between the side panels 26 and also attached to the supportmember 28. The inner panel 36 is provided with a circular throat opening38, and a speaker 40 suitable, for example, for reproducing sounds fromapproximately 40 cycles per second to 500 cycles per second is securedto the inner panel 36. An angular shaped member 42 is secured to thecover 30 to form a flaring passage from the front panel 24 to thesupport member 28, and a similar member 44 is secured to the bottom 32of the enclosure 22 for the same purpose; The enclosure here illustratedis a preferred construction, although the invention may be practicedwith other suitable speaker enclosures, or even without an enclosure.

A microphone 46 is disposed between the throat opening 38 of the speakerhousing 34 and the front panel 24 of the enclosure 22, said microphonebeing supported by suitable vibration isolators 48 and 50 which isolateit from mechanical vibrations occurring in the inner panel 36 or thefront panel 24 of the housing 34. In one embodiment, the isolators 48and 50 are resilient helical springs. The microphone 34 is electricallyconnected to the input of the low frequency amplifier 16 through a lowpass filter 52.

The high and low frequency separation circuit 18 is shown in detail inFigure 2. This circuit employs a single vacuum tube having two triodesections 54A and 5413. The grids 56 and 58 of these sections 54A and5413, respectively, of the vacuum tube are interconnected and alsoconnected to the preamplifier 20 through a capacitor 60. The grids 56and 58 are also connected to one of the positive terminals 62 of a powersource 64 through a resistor 66, and to the negative terminal 68 of thepower source 64, which is ground, through a resistor 70. The plates 72and 74 of the sections 54A and 54B are also interconnected and directlyconnected to the positive terminal 62 of the power source 64. Thecathode 76 of vacuum tube 54A is connected to the ground terminal 68through a resistor 78, and the cathode 80 of vacuum tube section 54B isconnected to this terminal 68 through a resistor 82.

A low-pass filter 84 is connected across the resistor '78. This low-passfilter 84 consists of four L-type resistor capacitance filters 86connected in cascade and connected to the cathode 76 of vacuum tubesection 54A by a condenser 88. Each of the L-type filters 86 has acondenser 90a, 90b, 900 or 90d and a resistor 92a, 92b, 920 or 92d,connected in series. The resistor 92a is connected to the condenser88,and the resistors 92a, 92b, 92c and 92d are connected in series, thejunctions of these resistors being resistor 242.

connected to ground by condensers a, 90b, 90c and 90d, respectively. Avoltage divider 94 is connected in parallel with condenser 90d of thefinal filter network 86, and the voltage divider 94 has a tap 96 whichis connected to the input of the low frequency amplifier 16.

A high-pass filter 98 is connected in parallel with the resistor 82.This filter 98 consists of four L-type condenser-resistor networks 100connected in cascade. Condenser 102a is connected to the cathode 80 andin series with condensers 102b, 1920 and 102d. The junctions betweenthese condensers are connected to ground by resistors 104a, 1041) and1040, respectively and a resistor 104d is connected from condenser 102dto ground and provided with a tap which is connected to the input of thehigh frequency amplifier 14.

In the particular construction of the invention described throughoutthis disclosure, the vacuum tube sections 54A and 54B are sections oftype 12AY7 tube, condenser 60 has a capacity of 0.25 mfd., resistor 70has a resistance of 100,000 ohms, resistor .66 a resistance of 600,000ohms, condenser 88 a capacity of 0.05 mfd., resistors 92a, 92b, 92c and92d resistances of 90,000 ohms, condensers 90a, 90b, 90c and 90dcapacities of 1235 mmfd., resistors 78 and 82 a resistance of 2150 ohms,resistor 94 a resistance of 1000 ohms, condensers 102a, 102b, 102a and102d have capacities of 0.0061 mfd., and resistors 104a, 104b, 104a and104d a resistance of 1000 ohms. The potential drop across the terminals62 and 68 of the power source 64 is volts.

The high frequency amplifier 14 has a voltage amplifier 200, phaseinverter 202, push-pull driver 204, and pushpull power amplifier stager206 connected in cascade. The high frequency speaker 10 is connected tothe output of the power amplifier 206, and the input of the voltageamplifier 200 is connected to the tap of resistor 104d in the high andlow frequency separation circuit 18.

The voltage amplifier 200 has three triode vacuum tube sections 208,210, and 212 connected in circuits which are connected in cascade.Vacuum tube section 208 has a grid 214 directly connected to the tap ofresistor 104d of the high and low frequency separation circuit 18. Thecathode 216 of vacuum tube section 208 is connected to the negativeterminal 68 of the power source 64 through a cathode resistor 218. Theplate 220 of vacuum tube section 208 is connected to the grid 222 ofvacuum tube section 210 through serially connected condensers 224 and226. The plate 220 of vacuum tube section 208 is also connected to apositive terminal 228 of the power supply 64 through a resistor 230, anda resistor 232 and condenser 234 connected in series are connected inparallel with the resistor 230. V

The grid 222 of vacuum tube section 210 is also connected to the groundterminal through a grid resistor 236, and a resistor 238 is connectedbetween the grid 222 and the junction between condensers 224 and 226.The cathode 240 of vacuum tube section 210 is also connected to theground terminal through a cathode The plate 244 of vacuum tube section210 is connected to the positive terminal 62 of the powers source 64through a plate resistor 246, and directly connected to the grid 248 ofvacuum tube section 212. A resistor 250 and condenser 252 connected inseries are also connected in parallel with resistor 246. The cathode 254of vacuum tube section 212 is connected to the ground terminal through aresistor 256 and the plate 258 of vacuum tube section 212 is connectedto a positive terminal 260 of the power source 64 through a resistor262.

In the voltage amplifier 200, the vacuum tube sections 208, 210 and2.12am sections of type 1ZAY7 tubes. Terminal 228 has a positivepotential of plus volts, and terminal 260 has a positive potential of380 volts with respect to the negative terminal 68 0f the power source.Resistor 218 has a resistance of 350 ohms, resistor 230 has a resistanceof 47,000 ohms, resistor 232 a resistance of 4,700 ohms, condenser 234 acapacity of 0.005 mfd., condenser 224 a capacitance of 0.1 mfd.,condenser 226 a capacitance of 0.004 mfd., resistor 238 a resistance of4,700 ohms, resistor 236 a resistance of 150,000 ohms, resistor 242 aresistance or" 910 ohms, resistor 246 a resistance of 47,000 ohms,resistor 250 a resistance of 4,700 ohms, condenser 252 a capacitance of0.0005 mfd, resistor 256 a resistance of 24,000 ohms, and resistor 262 aresistance of 24,000 ohms.

The phase inverter 202 employs two trinde vacuum tube sections 264 and266, section 264 having a grid of 268 connected to the plate 258 ofvacuum tube section 212 through serially connected condensers 270 and272. A resistor 274 is connected in parallel with resistor 272, and aresistor 276 is connected between the grid 268 and the ground terminal.Vacuum tube section 266 has a grid 278 which is connected to the cathode254 of vacuum tube section 212 through serially connected condensers 230and 282. A resistor 284 is connected in parallel with condenser 280, anda resistor 286 is connected between the ground terminal and the grid278. Vacuum tube section 264 has a plate 288 which is connected to thegrid 278 of vacuum tube section 266 through a condenser 290, and vacuumtube section 266 has a plate 292 connected through a condenser 294 tothe grid 268 of the vacuum tube section 264. Vacuum tube section 264 hasa cathode 296 connected to the ground terminal through a resistor 29S,and vacuum tube section 266 has a cathode 300 connected to the groundterminal through a resistor 302.

The driver stage 204 utilizes a vacuum tube having two triode sections304 and 306 connected in a pushpull cathode follower circuit. The vacuumtube sections 304 and 306 have plates 308 and 310, respectively, thatare interconnected and connected to a positive terminal 312 of the powersource 64. The tube sections 304 and 306 also have grids 314 and 316,respectively, the grid 314 being connected to the plate 288 of vacuumtube section 264 through coupling condenser 318, and the grid 316connected to the plate 292 of vacuum tube section 266 through couplingcondenser 320. The plate 288 of vacuum tube section 264 is connected toa positive terminal 322 of the power source 64 through series connectedresistors 324 and 326, and the plate 292 of vacuum tube section 266 isalso connected to the positive terminal 322 by a resistor 328 connectedto the junction of resistors 324 and 326. In addition, a resistor 330 isconnected in series with condenser 332, and these are connected inparallel with resistor 324; and a resistor 334 and condenser 336 areconnected in series, and these are connected in parallel with theresistor 328.

The vacuum tube sections 304 and 306 have cathodes 338 and 340 which areinterconnected by resistors 342 and 344, the junction between theresistors 342 and 34 being connected to a negative terminal 346 of thepower supply 64. The grids 314 and 316 are interconnected by a resistor348, voltage divider 350, and resistor 352 connected in series, and aresistor 354 is connected between the tap of the voltage divider 350 andthe junction of the resistors 34?. and 344. The junction betweenresistor 348 and the voltage divider 350 is connected to the groundterminal through a resistor 356 and a potentiometer 358, and thejunction between the voltage divider 350 and the resistor 352 isconnected to the junction between the resistor 356 and potentiometer 353by a resistor 360.

The power amplifier 206 employs a pair of vacuum tubes 362 and 364connected in a push-pull circuit. The vacuum tube 362 has a plate 366which is connected to one end of the primary winding 370 of an outputtransformer 372, the other end of the primary winding 370 beingconnected to a plate 374 of vacuum tube 364.

Vacuum tubes 362 and 364 are also provided with screen grids 376 and378, respectively, and the screen grids are connected to taps 380 and382, respectively, on the primary winding 310 of the transformer 372.The transformer primary winding 370 is also provided with a center tap384 which is directly connected to a positive terminal 312 of the powersource 64.

Vacuum tube 362 has a control grid 386 which is connected to the cathode338 of vacuum tube section 304 through a resistor 388; and vacuum tube364 has a control grid 390 connected to the cathode 340 of vacuum tubesection 306 through a resistor 392. The transformer 372 has a secondary394, one end of which is grounded, and the high frequency speaker 10 isconnected across the secondary 394 of the transformer 372. Vacuum tube362 has a cathode 410, and vacuum tube 364 has a cathode 412, thecathodes 410 and 412 being interconnected and connected to the groundterminal.

Two negative feedback systems are provided in the high frequencyamplifier 14. The first is provided from the plates 366 and 374 ofvacuum tubes 362 and 364 to the cathodes 296 and 300 of vacuum tubesections 264 and 266, respectively. These negative feedback circuitsconsist of a condenser 396 connected in series with a resistor 400between the plate 366 and the cathode 296; and a condenser 402 connectedin series with a resistor 404 between the plate 374 and the cathode 300.The second of these negative feedback systems consists of a condenser406 connected in parallel with the adjustable resistor 408 and thiscombination is connected between the ungrounded end of the secondary 394of the transformer and the cathode 216 of vacuum tube section 208 in thevoltage amplifier 200.

Vacuum tube 362 has a cathode 410, and vacuum tube 364 has a cathode412, the cathodes 410 and 412 being interconnected and connected to theground terminal.

In the exemplary construction of the invention described herein, vacuumtube sections 264 and 266 are sections of a 5814 tube, as are vacuumtube section 304 and 306. Vacuum tubes 362 and 364 are type KT-66 tubes.The potentials of the power source 64 measured with respect to theground terminal are: terminal 322 plus 380 volts, terminal 346 minusvolts, and terminal 312 plus 400 volts. The power supply 64 also has apair of terminals 414 which supply 6.3 volts D. C. which is connected tothe filaments of all of the tubes in the high and low frequencyseparation circuit 13, high frequency amplifier 14, and low frequencyamplifier 16, these connections being conventional and not illustrative.The following table sets forth the values of the condensers andresistors used in the phase inverter 202, driver 204, and poweramplifier stage 206.

Table 1 Resistor 262 ohms 24,000 Resistor 274 d0 1,000 Resistor 284 d01,000 Condenser 2'72 mfd 0.002 Condenser 230 mfd 0.002 Condenser 270 mfd0.1' Condenser 252 mfd 0.1 Resistor 276 ohms 150,000 Resistor 286 do150,000 Condenser 290 mmfd 1.5 Condenser 294 ummfdu 1.5 Resistor 29$ohms 2,500 Resistor 302 do 2,500 Condenser 332 mmfd 40 Resistor 323 ohms50,000 Resistor do 5,000 Resistor do 4,700 Petentioineter 350 do 10,000Resistor 404 d0 70,000

7 Table 1-Continued Condenser 396 rnfd 0.25 Resistor 400 ohms 70,000Condenser 402 mfd 0.25 Resistor 408 ohms 20,000 Condenser 406 mfd0.00025 Resistor 330 ohms 5,000 Resistor 324 do 50,000 Condenser 336mmfd 40 Resistor 326 ohms 5,000 Resistor 356 do 4,700 Variable resistor358 do 10,000 Resistor 352 do 150,000 Resistor 354 do s 4,700 Resistor342 do 24,000 Resistor 344 do 24,000 Resistor 388 do 1,000 Resistor 392do 1,000

The low frequency amplifier 16 employs a voltage amplifier 500, apush-pull cathode follower 502, a phase inverter 504, a voltageamplifier 506, a driver 508, and a push-pull power amplifier 510. Theoutput from the low frequency section of the high and low frequencyseparation circuit 18 is transmitted to the low frequency amplifier 16from the tap 96 of resistor 94 which is connected to a grid 512 of avacuum tube section 514 in the voltage amplifier 500. The vacuum tubesection 514 also has a cathode 516 which is connected to the groundterminal through series connected resistors 518 and 520. The plate 522of vacuum tube section 514 is connected to a positive terminal 524 of apower source 526 through a resistor 528. The resistor 528 is connectedin parallel with a resistor 530 and condenser 532 connected in series.

The push-pull cathode follower 502 employs a vacuum tube having a pairof triode sections 534 and 536. The two sections 534 and 536 have grids538 and 540, respectively, which are interconnected through -a doublepole single throw switch 542 normally positioned to form thisconnection. The grid 540 is also connected to the grid 538 through aresistor 544 when the switch 542 is in its other position for purposesof balancing the input circuit of the cathode follower 502. Grid 538 isalso connected to the plate 522 of vacuum tube section 514 through aresistor 545 and condenser 537 connected in parallel and a condenser549. The vacuum tube sections 534 and 536 are also provided withcathodes 546 and 548, respectively, and these cathodes areinterconnected through resistor 550, voltage divider 552 and resistor554 connected in series. The tap of the voltage divider 552 is connectedto the grid 538 through the rc sistor 554. The vacuum tube sections 534and 536 have plates 556 and 558, respectively, which are interconnectedand connected to a positive terminal 560 of the power source 526.

The phase inverter 504 is also provided with a vacuum tube having a pairof triode sections 562 and 564-. The cathode 566 of vacuum tube section562 is connected to the cathode 546 of vacuum tube section 534 through aresistor 538; and the cathode 570 of vacuum tube section 564 isconnected to the cathode 548 of vacuum tube section 536 through theresistor 572. The grid 573 of vacuum tube section 562 is directlyconnected to the cathode 548 of vacuum tube section 536; and the grid575 of vacuum tube section 564 is directly connected to the cathode 546of vacuum tube section 534. The plate 574 of vacuum tube section 562 isconnected to the positive terminal 560 through serially connectedresistors 576 and 578, and the junction of resistors 576 and 578 isconnected to the plate 580 of vacuum tube section 564 through a resistor582. A resistor 534 and a condenser 586 connected in series areconnected in parallel with the resistor 576; and a condenser 583 and aresistor 590 connected in series are also connected in parallel with theresistor 582.

The amplifier 506 has a vacuum tube with a pair of triode sections 592and 594. Vacuum tube section 592 has a grid 596 connected to the plate574 of vacuum tube section 562; and vacuum tube section 594 has a grid598 connected to the plate 580 of vacuum tube section 564. The cathodes600 and 602 of vacuum tube sections 592 and 594, respectively, areinterconnected by serially connected resistors 604 and 606, and thejunction of said resistors is connected to ground through a resistor608. Vacuum tube section 592 has a plate 610 which is connected to apositive terminal 612 of the power source 526 through serially connectedresistors 614 and 616, and vacuum tube section 594 has a plate 618connected to the junction of resistors 614 and 616 by a resistor 620.

The driver 508 employs a vacuum tube with a pair of vacuum tube sections622 and 624. Vacuum tube section 622 has a grid 626 connected to theplate 610 of vacuum tube section 592 through condenser 628, and vacuumtube section 624 has a grid 630 connected to the plate 618 of vacuumtube section 594 through a condenser 632. The grids 626 and 630 are alsointerconnected by serially connected resistor 634, potentiometer 636,and resistor 63- and potentiometer 636 is connected to the ground byserially connected resistors 640 and 642 and the junction betweenresistors 636 and 638 by a resistor 644. Vacuum tube section 622 alsohas a cathode 646 which is interconnected with the cathode 648 of vacuumtube section 624 by a pair of serially connected resistors 650 and 652,and the junction between the resistors 650 and 652 is connectedto thetap of resistor 636 by a resistor 654 and to a negative terminal 653 ofthe power supply 526.

The power amplifier 510 employs a pair of vacuum tubes 656 and 658connected in a push-null circuit. The grid 660 of vacuum tube 656 isconnected to the cathode 646 of vacuum tube section 622 through aresistor 662, and the grid 664 of vacuum tube 658 is connected to thecathode of 648 of vacuum tube section 624 through a resistor 666. Vacuumtube 656 has a cathode 668, and vacuum tube 65-? has a cathode 670, bothof these cathodes being connected to ground through a fuse 671. Vacuumtube 656 has a plate 672, and vacuum tube 658 has a plate 67 and theseplates are connected to the ends of the primary 676 of a transformer678. The primary 676 is provided with a center tap 680 which isconnected to a positive terminal 652 to the power source 526, thispositive terminal 632 also being connected to the plates 684 and 686 ofvacuum tube sections 622 and 624, respectively. The primary 676 of thetransformer 678 also has a pair of taps 688 and 690 which are connectedto the screen grids 65 2 and 694, respectively.

The transformer 678 has a secondary 696 which is connected to thespeaker 12 and to the ground terminal at one end. The other end of thesecondary 696 is connected to the junction of resistors 518 and 520 inthe voltage amplifier 500, thus forming a feedback loop, a parallelconnected condenser 698 and variable resistor 700 being seriallyconnected in the feedback loop. A resistor 702 connected to the plate672 of vacuum tube 656 and to the cathode 600 of vacuum tube section 592and a resistor [04 connected to the plate 674 of vacuum tube 658 and tothe cathode 6020f vacuum tube section 594 provide an additional feedbackloop from the power amplifier 510 to the amplifier 506.

In the particular construction of the invention described throughoutthis disclosure, vacuum tube section 1514 is one section of a type 12AY7 vacuum tube, vacuum tube sections 534 and 556 are portions of a type5814 vacuum tubc,'vacuum tube sections 562 and 564 are portions of atype 5751 vacuum tube, vacuum tube sections 592 and 594 are sections ofa type 5814.vacuum tube, vacuum tube sections 622 and 624 are sectionsof a type 5814 vacuum tube, and vacuum tubes 656 and 658 are type KT-66tubes. The power source 526 delivers plus 250 volts to terminal 524,plus 150 volts to terminal 560,

plus 400 volts to terminal 612, minus 105 volts to terminal 653, andplus 400 volts to terminal 682, all of these voltages being measuredrelative to ground and power supplies 64 and 526 having common grounds.In addition, the power source 526 hasterminals 706 which carry 6.3 voltsD. C. and are connected to the filaments inthe conventional manner.

The resistors and condensers employed in the low frequency amplifier 16are given in the following table.

Table II Resistor 52.8 -ohms 47,000 Resistor 530 do 5,000 Condenser 532rnfd 0.005 Condenser 549 mfd 0.1 Resistor 545 ohms 3,000 Condenser 547mfd 0.005 Resistor 544 hm's 1,000 Resistor 550 do 2,400 Resistor 552 do2,000 Resistor 554 do 2,400 Resistor 568 do 3,300 Resistor 572 do 3,300Resistor 578 do 24,000 Resistor 576 do 200,000 Resistor 582 do 200,000Resistor 584 do 22,000 Condenser 586 mfd 0.0001 Condenser 588 mfd 0.0001Resistor 590 0hms 22,000 Resistor 604 do 3,500 Resistor 606 d0 3,500Resistor 603 do 9,000 Resistor 614 do 100,000 Resistor 620 udo 100,000Resistor 616 do 10,000 Resistor 640 do 4,700 Resistor 644 do 4,700Resistor 64-2 do 10,000 Condenser 628 mfd 0.25 Condenser 632 mfd 0.25Resistor 634 ohms 510,000 Resistor 636 do 10,000 Resistor 63 8 do510,000 Resistor 654 do 4,700 Resistor 650 d0 24,000 Resistor 652 do24,000 Resistor 662 do 1,000 Resistor 666 do- 1,000 Resistor 702 do100,000 Resistor 704 d0 100,000 Condenser 698 I1lfd 0.00037 Resistor 700ohms 25,000

The microphone 46 is a crystal microphone and mounted in front of thethroat opening 38 in the speaker enclosure 22 at a distance no greaterthan 1 /2 feet from the vertex of the speaker cone. The microphone 46'is connected to the cathode 516 of the voltage amplifier 500 throughthe low pass filter 52 and a cathode follower 802. One terminal of themicrophone 46 is directly connected to the junction between resistors518 and 520 in the voltage amplifier 500, and the other terminal of themicrophone 46 is connected to a grid 804 of a vacuum tube section 806 inthe cathode follower circuit 802 through series connected resistors 808,810, and a portion of a voltage divider 812. The voltage divider 812 isconnected between the resistor 810 and the junction of resistors 518 and520. A resistor 814 is connected between the junction of resistors 808and 810 and the junction of resistors 518 and 520, and a resistor 816and condenser 820 connected in series are connected in parallel withresistor 814. In like manner, a resistor 822 and a condenser 824 areconnected in series and the combination is connected in parallel withthe voltage divider 812. 'It is thus clear that thefilter 52 is a stepfilter.

' In the particular construction described herein, vacuum tube section806 is one section of a type 12AY7 vacuum tube, the other section beingused in the voltage amplifier 500. The crystal microphone is ofconventional construction and is responsive to sounds having a frequencyfrom approximately 20 cycles per second to 12,000 cycles per second. Thesize of the various components in the low pass filter 52 are set forthin the following table.

Table III -Resistor808 ohms 1,000 Resistor 810 do 1,000 Resistor 814megohms 2.2 Resistor 816 ohms 500,000 Condenser 820 mfd 0.0006 Resistor822 ohms 180,000 Condenser 824 mfd 0.00015 Potentiometer 812 ohms 1,000

The vacuum tube section 806 has a cathode 826 which is directlyconnected to the cathode 516 of vacuum tube section 514. Vacuum tubesection 306 also has a plate 828 which is connected to the positiveterminal 560 of the power source 526.

The signal source impresses the electrical signals to be reproduced uponthe preamplifier 20 which amplifies the incoming signal without phaseshift and without changing the relative amplitude of the high frequencyand low frequency components of the signal. The preamplifier 20 has notbeen specifically disclosed, since many suitable pre-amplifiers havingflat frequency responses areavailable and well-known. The high and lowfrequency separations circuit effectively reduces the high frequencycomponents of the signal which is transmitted to the low frequencyamplifier 60 to negligible amplitude, the frequency range impressed uponthe low frequency amplifier 16 being approximately 30 cycles per secondto an upper limit of 500 cycles per second, and in the particularconstruction described herein this frequency range is from 30 cycles persecond to approximately 200 cycles per second. The upper limit of thefrequency range impressed upon the low frequency amplifier is determinedby the characteristics of the speaker enclosure 22, since speakerenclosures of the type described herein produce sounds having frequencyresponses with anupper limit falling off rapidly in the vicinity of 300to 500 cycles per second, depending upon the design of the enclosure.

The design of the high pass filter 98 of the high and low frequencyseparation circuit 18 is determined by the characteristics of the hi hfrequency amplifier 14 and high frequency speaker 10. In addition,however, the lower limit of the frequency range of the signals impressedupon the high frequency amplifier should approximately equal the upperlimit of the frequency range of the signals impressed upon the lowfrequency amplifier 16 inorder to cover the entire frequency range withthe two speakers 10 and 12.

The low frequency amplifier 16 has a flat frequency responseover therange of the signals impressed upon it from .the output of the low passfilter 84 of the high and low frequency separations circuit 18, and thusthe response of the low frequency speaker 12 also covers this range. Itis possible that a relatively high frequency oscillation could occur inthe feedback loop including the speaker 12, microphone 46, and lowfrequency amplifier 16, say in the vicinity of 1500 to 2500 cycles persecond, since in the design of the low frequency amplifier there is noreason to provide a cut-off at the high frequency end of its response. The low pass filter 52 attenuates the high frequency components of thesignals fed back in this feedback loop. This low pass filter 52 is astep type filter, and the filter must not introduce excessive phaseshift. For this reason, the maximum troll I off or attenuation of theoverall filter 52 must not eX- ceed db per octave, or excessive phaseshift will be introduced in this feedback loop. The filter 52 mustattenuate signals in excess of three to four times the crossoverfrequency to avoid oscillations.

As a result of introducing the feedback loop including the speaker 12,microphone 46, low pass filter 52, and low frequency amplifier 16, acleaner reproduction of the low frequency, or base, response in a highfidelity system is achieved. 'Further, no sensation of boominess occurs.

The man skilled in the art will devise many utilities and modificationsof the present invention beyond those specifically disclosed herein. Itis therefore intended that the scope of the present invention be notlimited by the foregoing disclosure, but rather only by the appendedclaims.

The invention claimed is:

1. A sound reproducing device comprising aspeaker arranged to operate upto a certain crossover frequency, an amplifier having an input adaptedto be connected to a signal source and an output connected to thespeaker, a microphone confronting the speaker at a distance from A; towavelength of said crossover frequency, and connected to the input ofthe amplifier, and means to at tenuate the frequency components producedby the microphone above several times said crossover frequency.

2. A sound reproducing device comprising a speaker arranged to operateup to a certain crossover frequency, an amplifier having an inputadapted to be connected to a signal source and an output electricallyconnected to the speaker, a microphone confronting the speaker andspaced therefrom by a distance ranging from /8 to A wavelength of saidcrossover frequency, and a low pass filter attenuating frequencies above1000 cycles per second electrically connected between the microphone andthe input of the amplifier.

3. A sound reproducing device comprising a high and low frequencyseparation circuit having an input adapted to be connected to an audiosource and high and low frequency outputs, said separation circuithaving a predetermined crossover frequency, a first amplifier having aninput electrically connected to the low frequency output of theseparation circuit, a first speaker connected to the first amplifier, asecond amplifier connected to the high frequency output of theseparation circuit, a second speaker connected to the second amplifier,a microphone confronting the first speaker and spaced therefrom between/s and A wavelength of said crossover frequency, and a low pass filterelectrically connected between the microphone and the input of the firstamplifier, said filter attenuating frequencies several times those ofsaid crossover frequency.

4. A sound reproducing device comprising a high and low frequencyseparation circuit having a selected crossover frequency including apair of cathode follower stages having vacuum tubes with interconnectedcontrol grids, interconnected plates, and cathodes, the first of saidstages having a plate to cathode circuit including a first resistor andthe second of said stages having a plate to cathode circuit including asecond resistor, a low pass filter connected across the first of saidresistors having a low frequency output, a high pass filter connectedacross the second of said resistors having a high frequency output, afirst amplifier having an input electrically connected to the lowfrequency output of the separation circuit, a first speaker connected tothe first amplifier, a second amplifier connected to the high frequencyoutput of the separation circuit, a second speaker connected to thesecond amplifier, a microphone confronting the first speaker and spacedtherefrom by /s to A wavelength of said crossover frequency, and a lowpass filter electrically connected between the microphone and the inputof the first amplifier, said filter attenuating frequencies above threeto four times said crossover frequency.

5. A sound reproducing device comprising a high and low frequencyseparation circuit operating relative to a certain crossover frequencyand having an input adapted to be connected to an audio source and highand low frequency outputs, a first amplifier having an inputelectrically connected to the low frequency output of the separationcircuit, a first speaker connected to the first amplifier, a secondamplifier connected to the high frequency output of the separationcircuit, a second speaker connected to the second amplifier, amicrophone confronting the first speaker and spaced therefrom by A; to 74 wavelength of said crossover frequency, and a low pass filterattenuating frequencies higher than said crossover frequency, saidfilter being electrically connected between the microphone and the inputof the first amplifier and having at least two low pass networksconnected in cascade, each of said networks including two resistors anda capacitor connected in series, and a third resistor con nected inparallel with the capacitor and one of the resistors, the following lowpass network being connected across the third resistor.

6. A sound reproducing device comprising a high and low frequencyseparation circuit operating relative to a certain crossover frequencyand having an input adapted to be connected to an audio source and highand low frequency outputs, a first amplifier having at its input 'a pairof stages, the second of said stages being a cathode follower stage,each stage including a vacuum tube with a plate, a grid, and a cathode,the cathodes of said tubes being interconnected and the grid of thefirst of said stages and an inner panel having a throat opening therein,a

r' first speaker mounted within the enclosure confronting the throatopening and facing the front panel, a second amplifier connected to thehigh frequency output of the separation circuit, a second speakerconnected to the second amplifier, a microphone disposed between thefront panel and the inner panel of the enclosure confronting the throatopening and electrically connected to the input of the first amplifier,said microphone being from the speaker a distance equal to A towavelength of said crossover frequency, and 'a low pass filterelectrically connected between the microphone and the grid of the secondstage of the first amplifier, said filter attenuating frequencies abovethree to four times the crossover frequency. e

7. A device for reproducing sound from a signal source comprising aspeaker enclosure including a front panel and an inner panel spacedrearwardly of said panel and having a throat opening therein, a speakermounted rearwardly of the opening in the enclosure, an amplifier havingan input adapted to be connected to a signal source and an outputconnected to the speaker, a microphone disposed between the front paneland the inner panel confronting the throat opening and electricallyconnected to the input of the amplifier, and means to attenuate'thefrequency components produced by the microphone above 1000 cycles persecond.

8. A device for reproducing sound from a signal source comprising aspeaker enclosure including a front panel, and an inner panel spacedrearwardly of said front panel andhaving a throat opening therein, aspeaker mounted rearwardly of the opening in the enclosure, an amplifierhaving an input adapted to be connected to a signal source and an outputconnected to the speaker, a microphone, vibration isolation meansattached to the front panel and the microphone and between the innerpanel and the microphone to mount said microphone between the frontpanel and the inner panel confronting the throat opening, and a low'pass filter attenuating frequencies above 1000 cycles per secondelectrically connected between the microphone and the input of theamplifier.

V 9. In a sound reproducing system having a plurality of loud speakers,the combination of high and low freasansa-s quency separation circuitshaving a certain crossover frequency, an amplifier connected to saidcircuits, a low frequency speaker connected to said amplifier, amicrophone positioned in front of said speaker a distance equal to A1 towavelength of said crossover frequency, and a low frequency filterconnected between said microphone and the input to said amplifier, saidfilter precluding the transmission of frequencies greater than fromthree to four times said crossover frequency.

10. In a sound reproducing system having a plurality of loud speakers,the combination of high and low frequency separation circuits having acertain crossover frequency, an amplifier connected to said circuits, alow frequency speaker connected to said amplifier, a microphonepositioned in front of said speaker a distance equal to A; to /4Wavelength of said crossover frequency, a negative feedback circuitinterconnecting said microphone and the input of said amplifier, saidlatter circuit including means for effectively eliminating frequenciesappreciably higher than said crossover frequency.

Toulon Sept. 8, 1931 Wilhelm Mar. 19, 1940

